Elemental sulfur is a vital raw material which is used in various industrial processes. For example, sulfuric acid production and vulcanization of rubber require large amounts of elemental sulfur. Most elemental sulfur is recovered from underground deposits using the thermally inefficient Frasch process or recovered as a by-product of natural gas and petroleum processing. However, underground deposits are dwindling and natural gas and petroleum production in the United States is on the decline. This has lead to concern over the future availability of elemental sulfur. In view of this, sulfur containing gaseous emissions from various industrial processes are beginning to be exploited as sources of elemental sulfur.
Processes are available for producing elemental sulfur from the hydrogen sulfide in coal gas, natural gas or oil refinery sour gas, such as the Stretford process. The Stretford process is a low temperature, wet oxidative scrubbing process. However, the Stretford process has numerous mechanical and process chemistry problems which include odor emissions, sulfur plugging in the towers, slurry basin sulfur deposition, and poor hydrogen sulfide removal efficiencies. Other similar processes use aqueous or glycol media which necessarily limits their operating temperature. Certain dry bed processes are commercially available; however, such processes are temperature limited to below the vaporization temperature of sulfur.
The most common process used to convert hydrogen sulfide to elemental sulfur has been that based on Claus technology. Generally, a preconcentration step is required. The process then oxidizes a portion of the feed gas to sulfur dioxide and reacts it with the nonoxidized portion to yield sulfur and water. Catalysts are used to ensure equilibrium at hourly gas space velocities limited to 700 to 1400 scf/cfh. The water generated in the equilibrium reaction limits the reaction. The ratio of hydrogen sulfide to sulfur dioxide must be strictly controlled as well the excess air. Temperatures are limited to about 600.degree. F. to avoid degradation of the catalyst. Reactions are equilibrium controlled so that the Claus process can only operate on concentrated hydrogen sulfide streams and produces a tail gas which requires further treatment. To overcome the equilibrium problem, various tail-end processes must be tacked onto the Claus, for example Claus/Beavon/IFT or Claus/Beavon/Scot or Claus/CBA. Even the newer SuperClaus process requires multi-staging and in any event require hydrogen sulfide as the starting compound. Thus, the Claus technology is complicated, sensitive and expensive.
Still more complex are those regenerable processes such as the Wellman-Lord which converts a very dilute sulfur dioxide gas stream, for example from combustion flue gas, to sulfuric acid or elemental sulfur. The Wellman-Lord process uses sodium sulfite to scrub the flue gas. The bisulfite product is thermally decomposed to yield a concentrated SO.sub.2 gas stream for sulfuric acid production. Partial reduction of the concentrated SO.sub.2 can produce elemental sulfur using augmented Claus technology at the tail end of the process. Another process for the treatment of flue gases is the NOXSO process which uses a reuseable sodium alumina sorbent. The sodium content of the catalyst is near 3 percent and reactivity of the sorbent is limited. In the NOXSO process, the sorption takes place near 244.degree. F. and the bed is then heated to temperatures above 1022.degree. F. for regeneration in a reducing gas. Both the Wellman-Lord type process and the NOXSO process are cyclical in regenerating the sorbent, complex and energy intensive.
Thus, it will be recognized that conventional methods for converting gaseous sulfur to elemental sulfur are typically complex, multistage processes. Also, conventional methods generally are operable at either low temperatures or extremely high temperatures, and often concentration of the sulfur containing gases is required.
Therefore, it is an object of the present invention to provide a method for producing elemental sulfur from sulfur containing gases at high gas space velocities.
It is another object of the present invention to provide a method for producing elemental sulfur from a gas stream containing reduced and/or oxidized sulfur compounds in a direct, continuous process.
A further object of the present invention is to provide a method for producing elemental sulfur from sulfur-containing gases which operates at intermediate temperatures.
Yet another object of the present invention is to provide a method for producing elemental sulfur from sulfur-containing gases which operates efficiently where concentrations of gaseous sulfur in the sulfur-containing feed gas are low.
Another object of the present invention is to provide a method for producing elemental sulfur from sulfur containing gases which can also be used to convert oxidized gaseous sulfur compounds to reduced sulfur compounds and vise versa.